I. Field of the Invention
The present invention relates to feedback control systems, and particularly, to a successive approximation control circuit for maintaining a set value of photo-receptor charge in an electrostatographic photo-copier thereby to produce uniform copies.
II. Description of the Prior Art
In many photo-copier applications, the copying process involves: (1) depositing a uniform layer of charge at a specified charge density on a surface, the surface being of the type which discharges when exposed to light; (2) exposing the surface to light; (3) attracting a supply of dark powderlike printing material to the charge image remaining on the surface after exposure to light; (4) transforming the powder image to paper if the charged surface was not initially sensitized paper; (5) bonding the powder into the paper, thus making a permanent copy.
The scope of this invention deals with step (1) of the above process: i.e., depositing charge on a photo-sensitive surface hereinafter referred to as a photoreceptor, or PR.
The PR is usually charged by passing a wire, a charge corotron, maintained at high voltage, near and over the surface thereof. This high voltage causes a current to flow thus charging the distributed capacitance of the PR surface.
The prior art has produced such power supplies, i.e., charge corotron power supplies, which are essentially high voltage, DC constant current regulated devices. The prior art, however, assumes that the deposited charge, once set, will remain at the correct level for producing uniform copies. Normally, the method is to set a fixed current; a better procedure is to manually adjust the power supply until the proper voltage (proportional to PR charge) is registered on an electrometer positioned near the PR but not at the same location as the charge corotron. Because the devices are not at the same place on the PR at the same times, a trial and error method is used involving: setting current; rotating the PR; checking electrometer voltage; and repeating until the proper voltage results. Unfortunately, this voltage would drift as the copy machine is used causing copy quality to degrade. The fallacy in the assumption that deposited charge will remain at the correct level is, at least in part, due to the fact that PR capacitance has a leakage component which is very sensitive to temperature and other influencing factors. It is this increase in leakage that causes less charge to appear on the PR at the time of powder pickup and which results in light copies.
It is desirable therefore to provide a system capable of self adjustment to compensate for changing values of a sensed variable which can not be controlled at the same time they are being sensed.
Accordingly, it is an object of the present invention to provide a sample data control circuit capable of maintaining a set value of PR charge in a photocopier regardless of the non-linearity of the PR capacitance, or leakage thereof, as affected by environmental changes.